The present invention generally relates to an acoustic coupler and a data transmission apparatus using the acoustic coupler.
Recently, there has been used a facsimile machine in which an acoustic coupler is set to the handset of the telephone set or vice versa so that a facsimile communication using a public telephone line can be easily realized. The acoustic coupler is set to the handset in such a way that a speaker-side portion of the acoustic coupler is coupled to a transmitter part of the handset, and a microphone-side portion of the acoustic coupler is coupled to a receiver part of the handset. In this state, the acoustic coupler is secured to the handset by a band or the like. Rubber packing members are respectively provided on the speaker-side portion and the microphone side-portion of the acoustic coupler so that a signal sound is prevented from leaking out of the acoustic coupler and a background noise is prevented from inputting to the acoustic coupler.
When a facsimile machine sends data on a document to a destination facsimile machine by using the acoustic coupler, an operator calls the destination facsimile machine by dialing and then sets the handset to the acoustic coupler after confirming the sound of a called station identification signal (CED signal). Then, the operator activates the facsimile machine, so that it is connected to the selected telephone line. When the destination facsimile machine sends the CED signal to the source facsimile machine, the fee accounting operation is started. Thus, the operator must set the handset to the acoustic coupler as soon as possible after confirming the CED signal sound. Unless the source facsimile machine responds to the CED signal within 30 seconds, for example, after receiving the CED signal, the destination facsimile machine determines that an error take places and stops the facsimile communication.
However, there are some cases where it takes a long time the operator to set the handset to the acoustic coupler. This occurs frequently, when facsimile communication is carried out by using an automobile telephone. In order to eliminate the above-mentioned problem, it is possible for the operator to set the handset to the acoustic coupler when calling the destination party so that the handset is spaced slightly apart from the acoustic coupler. However, this arrangement prevents the operator from hearing the CED signal sound. In addition, the handset is tightly fastened to the acoustic coupler during facsimile communication. Thus, there is a possibility that the operator does not notice the end of facsimile communication. In this case, the wasteful fee will be charged.